It is good to give. But the motive for what we do will determine our reward. Religious people in Jesus' time often liked to be seen as generous patrons of the poor.
After His extended discussion on prayer, Jesus returns to another way in which some religious people had become actors (hypocrites) and not real people of faith.
We treasure whatever is valuable to us; but they can soon be taken away. Thieves can steal them, water can rust them, small animals and insects can eat them.
The eye receives light from outside and transmits its message inside the person. Jesus said that the eye is like a lamp which shines inwards and the heart is illuminated by what the eye sees.
Fear comes to every sinner because fear is our response to the prospect of punishment (1 John 4:18). It is therefore natural to be afraid - which we most commonly exhibit as worry.